1. Introduction to Photochemical Smog Chemistry
Basic Reactions that form O3
Distinguish between O3 formation in the troposphere and stratosphere
How hydrocarbons and aldehydes participate in the formation of smog ozone
Formation of free radicals
Nitrogen loss mechanisms
Running simple simulation models

2. Why are we interested in the Smog Chemistry???

3. Ozone
ozone is a form of oxygen; it has three atoms of oxygen per molecule
It is formed in the lower troposphere (the atmosphere we live up to 6 km) from the photolysis of NO2
NO2 + light --> NO + O.
O. + O > O3 (ozone)
its concentration near the earth’s surface ranges from 0.01 to 0.5 ppm

4. Ozone background ranges from 0.02 to 0.06 ppm What is a ppm??
A ppm in the gas phase is one molecule per 106 molecules air or
1x10-6 m3 O3 per 1 m3 air or
1x10-6 atmospheres per 1 atmosphere of air
A ppm in water is 1x10-3grams /L water

5. Ozone let’s convert 1 ppm ozone to grams/m3
start with: 1x10-6 m3 per 1 m3 air
we need to convert the volume 1x10-6 m3 of O3 to grams
let’s 1st convert gas volume to moles and from the molecular weight convert to grams
at 25oC or 298K one mole of a gas= 24.45liters or 24.45x10-3 m3

6. Ozone we have 1x10-6 m3 of ozone in one ppm
so: 1x10-6 m = #moles O x10-3 m3/mol
O3 has a MW of 48 g/mole
so # g O3 in 1ppm = #moles Ox 48g/mole per m3
= 4.1x g/m3

7. Ozone Health Effects
Ozone causes dryness in the throat, irritates the eyes, and can predispose the lungs to bacterial infection.
It has been shown to reduce the volume or the capacity of the lungs for air
School athletes perform worse under high ambient O3 concentrations, and asthmatics have difficulty breathing
The current US standard has been just reduced from 0.12 ppm for one hour to 0.08 ppm for one hour

8. Lung function before exposure to O.32 ppm O3

9. Lung function after exposure to O.32 ppm O3

10. Athletic performance

11. How do we measure Ozone
40 years ago chemists borrowed techniques that were developed for water sampling and applied them to air sampling
for oxidants, of which O3 is the highest portion, a technique called “neutral buffered KI was used.
a neutral buffered solution of potassium iodide was placed in a bubbler

12. How do we measure Ozone
a neutral buffered solution of potassium iodide is placed in a bubbler
KI + O3 --> I2
measure I2

13. How do we measure Ozone
a neutral buffered solution of potassium iodide is placed in a bubbler
KI + O3 --> I2
measure I2
KI solution

14. How do we measure Ozone
A top is added to the bubbler so that air can enter the KI solution
KI + O3 --> I2
measure I2
KI solution

15. How do we measure Ozone a pump is attached to the bubbler pump
KI solution

16. How do we measure Ozone
Air goes in through the top of the bubbler and oxidants are trapped in the KI liquid and form I2
Air goes in
KI solution + I2

17. How do we measure Ozone
The absorbance of the I2 in the KI solution is then measured with a spectrophotometer
KI solution + I2

18. How do we measure Ozone
The absorbance of the I2 in the KI solution is then measured with a spectrophotometer
KI solution + I2

19. How do we measure Ozone
The absorbance of the I2 in the KI solution is then measured with a spectrophotometer
Spectrophotometer
KI solution + I2

20. A calibration curve
A standard curve is constructed from known serial dilutions of I2 in KI solution
to do this I2 is weighed out on a 4 place balance and diluted with KI solution to a known volume

21. A calibration curve
A standard curve is constructed from known serial dilutions of I2 in KI solution
to do this I2 is weighed out on a 4 place balance and diluted with KI solution to a known volume I2

22. Serial dilutions from stock solution

23. Serial dilutions from stock solution5
mg/Liter

24. Serial dilutions from stock solution5 3
mg/Liter

25. Serial dilutions from stock solution5 3 2
mg/Liter

26. Serial dilutions from stock solution5 3 2 1
mg/Liter

27. absorbances are measured for each of the serially diluted standards
Spectrophotometer
absorbance

28. Making a plot
I2 adsorbances are plotted vs. concentration

29. We then compare our sample absorbance to the standard curve
I2 absorbances are plotted vs. concentration
absorbance
air sample
concentration (mg/liter)

30. Problems
anything that will oxidize KI to I2 will give a false positive response
NO2, PAN, CH3-(C=O)-OO-NO2, give positive responses
SO2 gives a negative response

31. Instrumental techniques of measuring Ozone
Chemilumenescene became popular in the early 1970s
For ozone, it is reacted with ethylene
ethylene forms a high energy state of formaldehyde, [H2C=O]*
[H2C=O]*--> light + H2C=O
A photomultiplyer tube measures the light
The amount of light is proportional O3

32. Chemilumenescence measurement of Ozone
pump
sample air with O3 O3 PM
tube
{H2C=O}*
waste ethylene
ethylene
catalytic converter
CO2 + H2O

33. Using UV photometry to measure Ozone
This is the most modern technique for measuring ozone
sample air with O3 enters a long cell and a 254 nm UV beam is directed down the cell.
at the end of the cell is a UV photometer which is looking at 254 nm light
we know that: light Intensityout= light intensityin e- a LC

34. Photochemical Reactions
Oxygen (O2) by itself does not react very fast in the atmosphere.
Oxygen can be converted photochemically to small amounts of ozone (O3). O3 is a very reactive gas and can initiate other processes.
In the stratosphere O3 is good, because it filters uv light. At the earth's surface, because it is so reactive, it is harmful to living things

35. In the stratosphere O3 mainly forms from the photolysis of molecular oxygen (O2)
O2 + uv light -> O.
O. + O2 +M --> O3 + M
In the troposphere nitrogen dioxide from combustion sources photolyzes
NO2 + uv or visible light -> NO + O.
O. + O2 +M --> O3 (M removes excess energy and stabilizes the reaction)

36. O3 can also react with nitric oxide (NO)
O3 + NO -> NO2 + O2
both oxygen and O3 photolyzes to give O. O2 + hn -> O. +O. (stratosphere) O3 + hn -> O. + O2
O. can react with H2O to form OH. radicals O. + H2O -> 2OH.

37. OH. (hydroxyl radicals) react very quickly with organics and help “clean” the atmosphere; for example:
OH. + H2C=CH2 -> products ;very very fast
If we know the average OH. radical concentration, we can calculate the half-life or life time of many organics [org] in the atmosphere.

38. from simple kinetics we can show that:. d[org]/dt = -krate [org] [OH]
from simple kinetics we can show that: d[org]/dt = -krate [org] [OH] If [OH.] is constant
ln [org]t = ln [org]t=o -krate[OH.]x time1/2
Let’s say we want to know the time it takes for the organic to go to 1/2 its original [conc].

39. ln [org]t = ln [org]t=o -krate[OH.]x time1/2
rearranging ln {[org]t / [org]t=o }= -krate[OH.]x t1/2
The time that it takes for the conc to go to half means [org]t will be 1/2 of its starting conc. [org]t=o .
This means [org]t / [org]t=o = 1/2
and ln (1/2) = = -krate[OH.]x t1/2

40. if we use CO as an example, it has a known rate constant for reaction with OH.
CO + OH. -> CO krate= 230 ppm-1 min-1
If the average OH. conc. is 3 x10-8 ppm
for t1/2 we have: ln(1/2) = -krate[OH.] x t1/2
-0.693= -230 ppm-1 min-1 x 3 x10-8ppm x t1/2
t1/2 = min or 69.7 days

41. What this means is that if we emit CO from a car, 69
What this means is that if we emit CO from a car, 69.7 days later its conc. will be 1/2 of the starting amount. In another 69.7 days it will be reduced by 1/2 again.
For the same average OH. conc. that we used above, what would be the t1/2 in years for methane and ethylene, if their rate constants with OH. radicals are 12.4 and 3840 ppm-1 min-1 respectively? CH H2C=CH2

42. Why is the reaction of OH
Why is the reaction of OH. with ethylene so much faster than with methane? H H 1. H-C-H....OH . -> H-C. + .H OH H H H2C=CH2 attack by OH.is at the double bond, which is rich in electrons

43. What happens in urban air??
In urban air, we have the same reactions as we discussed before
NO2 + uv light -> NO + O.
O. + O2 +M --> O3 + M
O3 + NO -> NO2 + O2
This is a do nothing cycle

44. What is the key reaction that generates ozone at the surface of the earth?
What is the main reaction that generates it in the stratosphere?
How would you control O3 formation?

45. In the urban setting there are a lot of ground base combustion sources
Exhaust
hydrocarbons NO & NO2 CO

46. If organics are present they can photolyze or generate radicals
H2C=O + hn -> .HC=O + H.
H. + O2 -> .HO2
if we go back to the cycle NO2 + uv light -> NO + O. O. + O2 +M --> O3 + M O3 + NO -> NO2 + O2
.HO2 can quickly oxidize NO to NO2
NO + .HO2 -> NO2 + OH.

47. OH. + can now attack hydrocarbons which makes formaldehyde and other radical products
for ethylene CH2=CH2 + OH. -> OHCH2CH2. OHCH2CH2. + O2 -> OHCH2CH2O2.
OHCH2CH2O NO ->NO2 + OHCH2CH2O.
OHCH2CH2O. + O2 -> H2C=O + .CH2OH
O2 + .CH2OH -> H2C=O + .HO2

48. These reactions produce a host of radicals which “fuel” the smog reaction process
First OH radicals attack the electron rich double bond of an alkene
Oxygen then add on the hydroxy radical forming a peroxy-hydroxy radical
the peroxy-hydroxy radical radical can oxidize NO to NO2 ,just like HO2 can

49. Further reaction takes place resulting in carbonyls and HO2 which now undergo further reaction; the process then proceeds…

50. There is similar chemistry for alkanes
OH. + H 3-C-CH3 --> products
and for aromatics
OH. + aromatics --> products

51. Aromatic Reactions * toluene ring cleavage benzaldehyde o-cresolCH 3 OH 2 .
O=CH NO +O H * O
+ HO
+ H +
toluene
o-cresol
benzaldehyde
rearrangement
methylglyoxal
butenedial
oxygen bridge
radical ?
ring cleavage
Aromatic Reactions

52. Nitrogen Storage (warm vs. cool)OH H
C-C=O
+ H2O. H
C-C=O 3 . 3 H
PAN
warm
cool

53. Nitrogen Loss (HNO3 formation)
NO2 + O3  NO3.+ O2
NO3.+ NO2  N2O5
N2O5 + H2O  2HNO3 (surface)
NO2 + OH.  HNO3 (gas phase)

54. Nitrogen Loss (alkylnitrates)
butane O 2
-C-C-C-C-
2-butylnitrate NO
-C-C-C-C-
-C-C-C-C-
+ H. 2
2-butanal

55. How can we easily estimate O3 if we know NO and NO2?
The rate of of formation of O3 is governed by the reaction:
NO2 + uv light -> NO + O. and its rate const k1 because:
O. + O2 +M --> O3 + M is very fast
so the rate of formation O3 is:
rateform = +k 1 [NO2]

56. The rate of removal of O3 is governed by the reaction:
O3 + NO -> NO2 + O2 and its rate const k3
so the rate of removal of O3 is:
rateremov = -k 3 [NO] [O3]
the overall ratetot =rateform +rateremov

57. ratetot = -k 3 [NO] [O3] +k 1 [NO2]
if ratetot at steady state = 0, then
k 1 [NO2]= k 1 [NO2][O3] and
[O3] = k 1 [NO2] / {k 1 [NO2] }
This means if we know NO, NO2, k1 and k3 we can estimate O3

58. Calculate the steady state O3 from the following:. NO2 = 0. 28 ppm
Calculate the steady state O3 from the following: NO2 = 0.28 ppm NO = ppm k1 = 0.4 min-1 k3 = 26 ppm-1min-1

59. What is the key reaction that generates ozone at the surface of the earth?
What is the main reaction that generates it in the stratosphere?
How would you control O3 formation?

60. Can we use computers to predict the amount of ozone formed if we know what is going into the atmosphere?
yes
but we need to create experimental systems to see of our models are working correctly.

61. In 1972 we built the first large outdoor smog chamber, which had an interior volume of 300 m3.
We wanted to predict oxidant formation in in the atmosphere.
The idea was to add different hydrocarbon mixtures and NO + NO2, to the chambers early in the morning.

62. Samples would be taken through out the day
Samples would be taken through out the day. We would then compare our data to the predictions from chemical mechanisms.
If we could get a chemical mechanism to work for many different conditions, we would then test it under real out door- urban conditions.

63. The Chamber had two sides
Or Darkness
NO &NO2
300 m3 chamber
Teflon Film walls
propylene
Formaldehyde

64. Example experiment with the following chamber concentrations:
NO = 0.47
NO2 = 0.11 ppm
Propylene = 0.99 ppmV
temp = 15 to 21oC

65. Solar Radiation Profile

66. Example Mechanism dNO2/dt = -k1[NO2]; DNO2=-k1 [NO2] Dt
NO2+ hn -> NO + O k1 keyed to sunlight
O. + O > O k2
O3 +NO2 --> NO + O3 k3
H2C=O + hn --> .HC=O + H. k4 keyed to sunlight
H. +O > HO k5
HO2. + NO --> NO2+OH. k6 (fast)
OH.+ C=C ---> H2C=O + HO H2COO. k7
dNO2/dt = -k1[NO2]; DNO2=-k1 [NO2] Dt

67. Photochemical System

68. Photochemical System

69. let’s see how this kinetics model works
1st we will look at a mechanism
2nd we will look at the model inputs
3rd we will run the model with reduced hydrocarbons (formaldehyde) to see the effect of reducing HC
run the model with reduced NOx

72. We need to get mixing height into the model
height in kilometers
0.0
0.1
0.2
0.3
0.4
1.1
1.5
Balloon temperature
Temp in oC 20 25 30 35
Dry adiabatic
lines

73. How do we get light into the mechanism??
A molecule photolyzes or breaks apart when it absorbs photons that have energy that is greater than the bond strength
Let’s look at the energy in a mole of photons which have a wavelength 288 nm
The energy E, in a mole of light at 288 nm is
E= 6.02x1023x hc/l c= 3x108m/s; h=6.63x10-34Js, l=288x10-9m
E= 416kJ/mole
If all this light was absorbed it would break C-H bond

74. Light and rate constants
The question is, is all the light absorbed??
Actually not, but this brings up the concept of quantum yields, f, and light absorption s
f= # molecules reacted/# photons absorbed
The light flux is the # of photons of light cm-2 sec-1
I have called this I but often it is called J or F at a given l
The rate constant for photolyis can be written as kratel= Jl x f l x absorption coefl

75. Light and rate constants
kratel= Jl x f l x absorption coefl
the absortion coef. s has units of cm2/molecule and comes from Beer’s law I=Io e-sl[C]
kratel= Jl1 x f l1 x s l1
This is at one wavelength l1; what do we do when we have two wavelengths l1 and l2?
krateltotal = Jl1 x f l1 x s l1 + Jl2 x f l2x s l2

76. Light and rate constants
so across all wavelengths
krateltotal = S Jl x f l x s l
What this says is that if we know the light flux or “intensity” at each wavelength, Jl , the absorption coef., s l at each wavelength and the quantum yield s l , we can calculate krateltotal for the real atmosphere

77. Light and rate constants
Lets calculate kratel for NO2 at the wave length of nm and a zenith angle of 20 degrees
J nm= photons cm-2 sec-1 = x1015
f400nm = quantum yield = ~0.65
s nm = ~6x10-19 cm2 molecule-1
kratel= Jl x f l x s l= sec-1

80. Light and rate constants
so in the reaction
NO2 + light at nm -> NO + O.
kratel= Jl x f l x s l= sec-1
dNO2/dt = krate l [NO2]

81. Solar flux striking the earth

82. Light and rate constants
The angle of the sun is called the zenith angle. When the sun is directly over head the zenith angle is zero degrees
when it has just gone down it is 90o

83. Light and rate constants (zenith angle)
Sun q

84. Light and rate constants
When the sun is at lower angles in the sky, less energy strikes the surface of the earth; sun up and sun down the zenith angle is zero
Algorithms have been written to relate zenith angle to light flux at each wavelength

86. Light and rate constants
The zenith angle for given latitude and time of year can known, as well as the time that the sun comes up and how high in the sky it will go at noon
in the winter time it will not go as high in the sky as in summer.

88. Light and rate constants
from these tables if we know f and s for a compound we can calculate the photolysis rate constants for any compound over the course of the day as the zenith angle changes
NO2, H2C=O, O3, acetaldehyde